> The problem is more or less solved - see Nix.

Sorry, but I'm really not sure what you mean by "solved".

Nix is yet another (language agnostic) package manager with certain tradeoffs. But, if there is not an available Nix package for a specific version of a library I need to use - I'm out of luck.

Nix is not a build tool designed to work with arbitrary or latest development versions of libraries, for example. And, it will never solve that problem even _if_ it is technically capable of doing so - because there is no force in the world that would get all projects in all languages to use it.